Screens are used for separating aggregate material according to size. One common form of screen is a grizzly which comprises a set of parallel bars in spaced apart relationship to provide elongate screen openings. In some situations, the screen openings in grizzlys can become clogged by material obstructing the openings. The obstructing material may be trapped in the screen openings or may simply accumulate on the intake side of the screen openings.
The present invention seeks to provide a screening device which has a screen and provision to remove obstructing material from screen openings within the screen.
The screening device according to the invention has particular application in the recovery of material from an underground location. In such an application, the screening device may be incorporated in a recovery head operable underground. Indeed, the recovery head incorporating the screening device may form part of an underground mining apparatus which has been devised particularly, although not necessarily exclusively, for mining in normally inaccessible locations including underground land-based formations such as deep leads, and deep marine alluvial deposits.
A deep lead typically comprises an ancient river valley through which an underground stream flows through an aquifer which contains metallic particles or heavy minerals below an overburden containing mud, sand and in some cases basalt. The metallic particles and/or heavy minerals in the stream bed are concentrated in the lower regions of sand and that part of the bedrock which has corroded to form a deposit known as "wash". The wash may be of irregular composition, comprising a combination of sand, mud, and rocks and boulders of various sizes. To efficiently recover the metallic particles and/or heavy minerals, only wash material should be removed. Removal of wash material does, however, present considerable difficulties owing to its irregular composition and the uneven nature of the bedrock beneath it. It is the presence of the rocks and boulders in the wash material which presents a particular difficulty.
The inaccessible nature of the formation and in particular the depth underground as well as the irregular composition of the wash material precludes mining by way of a conventional dredging process.
Similarly, marine deposits may be too deep for conventional mining operations.
The present invention also seeks to provide a novel and useful system for recovering materials from underground locations.